Plant Defence 101: Insect Reinforcements

Two weeks ago, I wrote about how plants can sense when they are being attacked by insects, right? Well, they can do more than just try to poison them by making nasty chemicals. Plants do in fact have a very nice, effective private army at their disposal: other insects!

Confused? Well, that is understandable. One moment I am talking about how insects damage plants, and the next I say that they are rescuing them. What?

The thing is, there are many, many species of insects. Some eat plants, like we discussed last time, but some others prey on fellow insects. And that last group, those are the helpers of the plants, their saviours.

So, when a plant detects some insects that are munching on its leaves, he will immediately send toxic compounds to the damaged areas, to make the insects stop eating and hopefully even get really ill. But many plants do something else in the meantime too. They can send out a signal for other insects to pick up on. That signal is usually in the form of a volatile substance. Let’s call it a “smell” for simplicity’s sake, although it is actually a bit different, smell is the human sense most closely resembling it.
Okay, the plant emits a certain smell. Sometimes the strength of the smell depends on the amount of damage the eating insects do, sometimes it is a fixed potency. That smell can be sensed by other (carnivorous) insects in the area, who will come to the rescue. They can start preying upon the plant-eating insects, to help the plant get rid of its attackers. Insects that do this are for example ladybugs.

Ladybugs to the rescue! Maybe this plant was suffering from an attack of Aphids

Sometimes they can help in other ways too, like laying their eggs inside other insect. Parasitoid wasps are a good example of that. They can use the plant signals to locate their host-insects, stab it with their angel and deposit their eggs right inside the insect. Caterpillars are among their favourite hosts. As soon as the eggs hatch, the larvae will start eating the caterpillar from the inside out. (I know, it sounds really creepy and gross, sorry guys!)
Needless to say, when the caterpillar is eaten, it won’t be damaging the plant any more.

Even before the caterpillar is dead, the parasitoid wasp can save the plant. In many cases, the presence of the eggs somehow triggers strange responses in the host insect, making it walk a lot, be aggressive to other species or do other crazy things, all apparently for the benefit of the parasitoid larvae. As soon as that happens, the host insect usually won’t eat any more.

The same thing goes for butterfly eggs, by the way. You know that plants can sense those as well, and it can just the same send out an alarm call to parasitoids in that case. The other insect (usually a parasitoid wasp) will lay its own eggs inside the eggs of the butterfly, effectively preventing them from ever hatching.

Some caterpillars have develloped defence traits to keep parasitoids away

Fascinating, right? How so much processes can build up on each other? Oh, and to make the confusion even worse, parasitoid wasps themselves can in turn be infected with eggs from other “hyperparasitoids”. Amazing!

Have you ever seen a caterpillar acting very strange? Do you think it could have been because of it being infected by a parasitoid?